1. Field of the Invention
The present invention relates to a two-cycle internal combustion engine, suited for use in a portable working machine, such as a chain saw, which is adapted to be operated in various postures.
2. The Prior Art
As a power source for a portable working machine, such as a chain saw, which is adapted to be operated in various postures, an air-cooled two-cycle gasoline internal combustion engine of small type (hereinafter referred to as a two-cycle internal combustion engine or simply as an engine) is usually employed. Since the size of such a two-cycle internal combustion engine is generally larger in the longitudinal direction (height) than in the lateral direction, the engine is generally arranged horizontally in a main case, as seen in the case of a small chain saw for instance.
In the case of the aforementioned two-cycle internal combustion engine which is adapted to be laid horizontally in the main case of a working machine, such as a chain saw, and particularly in the case of the engine as shown in Japanese Utility Model Unexamined Publication S/56-140402 (corresponds to U.S. Pat. No. 4,370,809), wherein a suction port is formed at the bottom (a forward portion when viewed in a laid posture) of a crankcase and an air-fuel mixture from a carburetor is sucked from a suction port via a lead valve to the crankcase to be pre-compressed therein, the resultant pre-compressed air-fuel mixture being transferred through a scavenging passage to a combustion chamber, there has been frequently experienced a phenomenon of extraordinary fluctuation of rotational speed or sudden stalling of the engine, resulting in the stoppage of the engine when the forward portion of the working machine (chain saw) is directed upward or obliquely upward after the working machine is operated while directing the forward portion thereof downward or obliquely downward for a period of time.
The cause for this phenomenon has been studied by the present inventors and made clear as follows. Namely, an unatomized raw fuel (a liquid fuel) which has adhered at first on the inner peripheral wall of a crank chamber of the engine collectively flows into the suction port to be accumulated therein throughout a period when the forward portion of the working machine is directed downward or obliquely downward, i.e. when the suction port which opens to the crankcase of the engine is directed downward or obliquely downward. However, when the forward portion of the working machine is directed upward or obliquely upward, this unatomized fuel that has been accumulated in the suction port is caused to flow into the scavenging passage through the inner peripheral wall of the crank chamber and then rush-flows into the combustion chamber from the scavenging passage, thereby supplying an excessively thickened air-fuel mixture to the combustion chamber for combustion. In other words, the cause for the phenomenon can be ascribed to an undesirable flow of unatomized raw fuel due to the change in posture of the engine. To date, however, no practically effective means has been provided to eliminate this undesirable phenomenon.
When the engine is in a state of high load and high rotational speed, the quantity of fuel per unit time is relatively large, so that even if the unatomized raw fuel is allowed to rush-flow into the combustion chamber as mentioned above, no serious inconvenience would be caused to occur though some degree of fluctuation in rotational speed may be caused to occur. However, when the engine is in a state of idling, the quantity of fuel per unit time is relatively little so that when the unatomized raw fuel is allowed to rush-flow into the combustion chamber, the air-fuel mixture becomes excessive in thickeness, thus giving rise to a serious problem, e.g. the stoppage of the engine.
In an attempt to solve this problem, the present inventors have proposed (See Japanese Patent Unexamined Publication H/9-151739) the installation of a flow control portion such as a linear projection, groove or recess at a portion of the inner peripheral wall of the crank chamber which is in the vicinity of the suction port for reducing the flow rate of the unatomized raw fuel.
One example of a two-cycle internal combustion engine provided with this flow control portion is illustrated in FIG. 4, which shows a cross-section of the main portion of the two-cycle internal combustion engine. This two-cycle internal combustion engine 100 comprises a carburetor 30 constituting means for forming an air-fuel mixture, a crankcase 20 provided with a crank chamber 22 and a suction port 25 opening to the crank chamber 22, an insulator 40 attached to the crankcase 20 and provided with a suction passage 42 for introducing the air-fuel mixture from the carburetor 30 to the suction port 25, and a reed valve 35 formed of a tab-shaped elastic piece whose proximal end portion 35a is fixed to the downstream side end face 40A of the insulator 40 facing the suction port 25 so as to allow a free end portion 35b of the tab-shaped elastic piece to be optionally press-contacted with the downstream side end face 40A of the insulator 40, thereby opening or closing the suction passage 42.
A pair of linear projections 23 (each functioning as a flow-controlling member for controlling flow speed of unatomized raw fuel), each being rectangular in cross- section and spaced apart from the other, are formed on the inner peripheral wall 22A of the crank chamber 22, traversing the whole width of the crank chamber 22, in close proximity to the suction port 25 and in parallel with a crank shaft rotatably supported in the crankcase 20.
According to the engine 100 constructed in this manner, even if the unatomized raw fuel which has been trapped at the suction port 25 tends to flow into the scavenging passage (the outside of the apparatus shown in FIG. 4) through the inner peripheral surface 22A of the crank chamber 22, the flow of the unatomized raw fuel is interrupted by the pair of linear projections 23 (each functioning as a flow-controlling member) formed on the inner peripheral wall 22A of the crank chamber 22 in close proximity to the suction port 25, thus resulting in a prominent slow down in the flow rate of the unatomized raw fuel.
Therefore, the possibility that the unatomized raw fuel would flow into the combustion chamber in a rush-flow manner through the inner peripheral wall of the crank chamber can be remarkably reduced. As a result, the possibility of a violent fluctuation of rotational speed of the engine or a sudden stalling or stoppage of the engine can be remarkably reduced.
However, even in this engine 100 provided as mentioned above with a flow controlling member, i.e. the linear projections 23 formed on the inner peripheral surface 22A of the crank chamber 22, a phenomenon of rush-flow of unatomized raw fuel into the combustion chamber has been occasionally recognized when the suction port 25 is suddenly directed upward or obliquely upward after the suction port 25 has been directed downward or obliquely downward for a long period of time. Namely, a large quantity of the unatomized raw fuel tends to be trapped at the stepped corner portion P which is located at the downstream side end face 40A of the insulator 40 below the free end portion 35b of the reed valve 35, and the resultant trapped unatomized raw fuel is sometimes caused to rush-flow, passing over the linear projections 23, into the combustion chamber. In other words, the provision of the aforementioned flow-controlling member is not sufficient to completely control the flow of the unatomized raw fuel, i.e. the aforementioned problem is not yet completely solved.